The Node.js Event Loop, Timers, and process.nextTick()

What is the Event Loop?

The event loop is what allows Node.js to perform non-blocking I/O
operations — despite the fact that JavaScript is single-threaded — by
offloading operations to the system kernel whenever possible.

Since most modern kernels are multi-threaded, they can handle multiple
operations executing in the background. When one of these operations
completes, the kernel tells Node.js so that the appropriate callback
may be added to the poll queue to eventually be executed. We'll explain
this in further detail later in this topic.

Event Loop Explained

When Node.js starts, it initializes the event loop, processes the
provided input script (or drops into the REPL, which is not covered in
this document) which may make async API calls, schedule timers, or call
process.nextTick(), then begins processing the event loop.

The following diagram shows a simplified overview of the event loop's
order of operations.

Each phase has a FIFO queue of callbacks to execute. While each phase is
special in its own way, generally, when the event loop enters a given
phase, it will perform any operations specific to that phase, then
execute callbacks in that phase's queue until the queue has been
exhausted or the maximum number of callbacks has executed. When the
queue has been exhausted or the callback limit is reached, the event
loop will move to the next phase, and so on.

Since any of these operations may schedule more operations and new
events processed in the poll phase are queued by the kernel, poll
events can be queued while polling events are being processed. As a
result, long running callbacks can allow the poll phase to run much
longer than a timer's threshold. See the timers and
poll sections for more details.

NOTE: There is a slight discrepancy between the Windows and the
Unix/Linux implementation, but that's not important for this
demonstration. The most important parts are here. There are actually
seven or eight steps, but the ones we care about — ones that Node.js
actually uses - are those above.

Phases Overview

timers: this phase executes callbacks scheduled by setTimeout()
and setInterval().

I/O callbacks: executes almost all callbacks with the exception of
close callbacks, the ones scheduled by timers, and setImmediate().

idle, prepare: only used internally.

poll: retrieve new I/O events; node will block here when appropriate.

check: setImmediate() callbacks are invoked here.

close callbacks: e.g. socket.on('close', ...).

Between each run of the event loop, Node.js checks if it is waiting for
any asynchronous I/O or timers and shuts down cleanly if there are not
any.

Phases in Detail

timers

A timer specifies the thresholdafter which a provided callback
may be executed rather than the exact time a person wants it to
be executed. Timers callbacks will run as early as they can be
scheduled after the specified amount of time has passed; however,
Operating System scheduling or the running of other callbacks may delay
them.

For example, say you schedule a timeout to execute after a 100 ms
threshold, then your script starts asynchronously reading a file which
takes 95 ms:

const fs =require('fs');functionsomeAsyncOperation(callback){// Assume this takes 95ms to complete
fs.readFile('/path/to/file', callback);}const timeoutScheduled = Date.now();setTimeout(()=>{const delay = Date.now()- timeoutScheduled;
console.log(`${delay}ms have passed since I was scheduled`);},100);// do someAsyncOperation which takes 95 ms to completesomeAsyncOperation(()=>{const startCallback = Date.now();// do something that will take 10ms...while(Date.now()- startCallback <10){// do nothing}});

When the event loop enters the poll phase, it has an empty queue
(fs.readFile() has not completed), so it will wait for the number of ms
remaining until the soonest timer's threshold is reached. While it is
waiting 95 ms pass, fs.readFile() finishes reading the file and its
callback which takes 10 ms to complete is added to the poll queue and
executed. When the callback finishes, there are no more callbacks in the
queue, so the event loop will see that the threshold of the soonest
timer has been reached then wrap back to the timers phase to execute
the timer's callback. In this example, you will see that the total delay
between the timer being scheduled and its callback being executed will
be 105ms.

Note: To prevent the poll phase from starving the event loop, libuv
(the C library that implements the Node.js
event loop and all of the asynchronous behaviors of the platform)
also has a hard maximum (system dependent) before it stops polling for
more events.

I/O callbacks

This phase executes callbacks for some system operations such as types
of TCP errors. For example if a TCP socket receives ECONNREFUSED when
attempting to connect, some *nix systems want to wait to report the
error. This will be queued to execute in the I/O callbacks phase.

poll

The poll phase has two main functions:

Executing scripts for timers whose threshold has elapsed, then

Processing events in the poll queue.

When the event loop enters the poll phase and there are no timers
scheduled, one of two things will happen:

If the poll queue is not empty, the event loop will iterate
through its queue of callbacks executing them synchronously until
either the queue has been exhausted, or the system-dependent hard limit
is reached.

If the poll queue is empty, one of two more things will
happen:

If scripts have been scheduled by setImmediate(), the event loop
will end the poll phase and continue to the check phase to
execute those scheduled scripts.

If scripts have not been scheduled by setImmediate(), the
event loop will wait for callbacks to be added to the queue, then
execute them immediately.

Once the poll queue is empty the event loop will check for timers
whose time thresholds have been reached. If one or more timers are
ready, the event loop will wrap back to the timers phase to execute
those timers' callbacks.

check

This phase allows a person to execute callbacks immediately after the
poll phase has completed. If the poll phase becomes idle and
scripts have been queued with setImmediate(), the event loop may
continue to the check phase rather than waiting.

setImmediate() is actually a special timer that runs in a separate
phase of the event loop. It uses a libuv API that schedules callbacks to
execute after the poll phase has completed.

Generally, as the code is executed, the event loop will eventually hit
the poll phase where it will wait for an incoming connection, request,
etc. However, if a callback has been scheduled with setImmediate()
and the poll phase becomes idle, it will end and continue to the
check phase rather than waiting for poll events.

close callbacks

If a socket or handle is closed abruptly (e.g. socket.destroy()), the
'close' event will be emitted in this phase. Otherwise it will be
emitted via process.nextTick().

setImmediate() vs setTimeout()

setImmediate and setTimeout() are similar, but behave in different
ways depending on when they are called.

setImmediate() is designed to execute a script once the current
poll phase completes.

setTimeout() schedules a script to be run after a minimum threshold
in ms has elapsed.

The order in which the timers are executed will vary depending on the
context in which they are called. If both are called from within the
main module, then timing will be bound by the performance of the process
(which can be impacted by other applications running on the machine).

For example, if we run the following script which is not within an I/O
cycle (i.e. the main module), the order in which the two timers are
executed is non-deterministic, as it is bound by the performance of the
process:

The main advantage to using setImmediate() over setTimeout() is
setImmediate() will always be executed before any timers if scheduled
within an I/O cycle, independently of how many timers are present.

process.nextTick()

Understanding process.nextTick()

You may have noticed that process.nextTick() was not displayed in the
diagram, even though it's a part of the asynchronous API. This is because
process.nextTick() is not technically part of the event loop. Instead,
the nextTickQueue will be processed after the current operation
completes, regardless of the current phase of the event loop.

Looking back at our diagram, any time you call process.nextTick() in a
given phase, all callbacks passed to process.nextTick() will be
resolved before the event loop continues. This can create some bad
situations because it allows you to "starve" your I/O by making
recursive process.nextTick() calls, which prevents the event loop
from reaching the poll phase.

Why would that be allowed?

Why would something like this be included in Node.js? Part of it is a
design philosophy where an API should always be asynchronous even where
it doesn't have to be. Take this code snippet for example:

The snippet does an argument check and if it's not correct, it will pass
the error to the callback. The API updated fairly recently to allow
passing arguments to process.nextTick() allowing it to take any
arguments passed after the callback to be propagated as the arguments to
the callback so you don't have to nest functions.

What we're doing is passing an error back to the user but only after
we have allowed the rest of the user's code to execute. By using
process.nextTick() we guarantee that apiCall() always runs its
callback after the rest of the user's code and before the event loop
is allowed to proceed. To achieve this, the JS call stack is allowed to
unwind then immediately execute the provided callback which allows a
person to make recursive calls to process.nextTick() without reaching a
RangeError: Maximum call stack size exceeded from v8.

This philosophy can lead to some potentially problematic situations.
Take this snippet for example:

let bar;// this has an asynchronous signature, but calls callback synchronouslyfunctionsomeAsyncApiCall(callback){callback();}// the callback is called before `someAsyncApiCall` completes.someAsyncApiCall(()=>{// since someAsyncApiCall has completed, bar hasn't been assigned any value
console.log('bar', bar);// undefined});
bar =1;

The user defines someAsyncApiCall() to have an asynchronous signature,
but it actually operates synchronously. When it is called, the callback
provided to someAsyncApiCall() is called in the same phase of the
event loop because someAsyncApiCall() doesn't actually do anything
asynchronously. As a result, the callback tries to reference bar even
though it may not have that variable in scope yet, because the script has not
been able to run to completion.

By placing the callback in a process.nextTick(), the script still has the
ability to run to completion, allowing all the variables, functions,
etc., to be initialized prior to the callback being called. It also has
the advantage of not allowing the event loop to continue. It may be
useful for the user to be alerted to an error before the event loop is
allowed to continue. Here is the previous example using process.nextTick():

When only a port is passed the port is bound immediately. So the
'listening' callback could be called immediately. Problem is that the
.on('listening') will not have been set by that time.

To get around this the 'listening' event is queued in a nextTick()
to allow the script to run to completion. Which allows the user to set
any event handlers they want.

process.nextTick() vs setImmediate()

We have two calls that are similar as far as users are concerned, but
their names are confusing.

process.nextTick() fires immediately on the same phase

setImmediate() fires on the following iteration or 'tick' of the
event loop

In essence, the names should be swapped. process.nextTick() fires more
immediately than setImmediate() but this is an artifact of the past
which is unlikely to change. Making this switch would break a large
percentage of the packages on npm. Every day more new modules are being
added, which mean every day we wait, more potential breakages occur.
While they are confusing, the names themselves won't change.

We recommend developers use setImmediate() in all cases because it's
easier to reason about (and it leads to code that's compatible with a
wider variety of environments, like browser JS.)

Why use process.nextTick()?

There are two main reasons:

Allow users to handle errors, cleanup any then unneeded resources, or
perhaps try the request again before the event loop continues.

At times it's necessary to allow a callback to run after the call
stack has unwound but before the event loop continues.

Say that listen() is run at the beginning of the event loop, but the
listening callback is placed in a setImmediate(). Now, unless a
hostname is passed binding to the port will happen immediately. Now for
the event loop to proceed it must hit the poll phase, which means
there is a non-zero chance that a connection could have been received
allowing the connection event to be fired before the listening event.

Another example is running a function constructor that was to, say,
inherit from EventEmitter and it wanted to call an event within the
constructor:

You can't emit an event from the constructor immediately
because the script will not have processed to the point where the user
assigns a callback to that event. So, within the constructor itself,
you can use process.nextTick() to set a callback to emit the event
after the constructor has finished, which provides the expected results: